The present invention relates generally to spread spectrum clock generators and is particularly directed to multiple spread spectrum clock generators of the type which exhibit overlapping output frequencies. The invention is specifically disclosed as a multiple output spread spectrum clock generator circuit that is mounted on a single substrate, in which the electromagnetic emissions are somewhat reduced by introducing and controlling a phase difference between operating frequencies of the multiple clock outputs, even though these operating frequencies overlap one another.
In typical clock generation circuits, emissions from multiple independent sources will add together at each frequency. This is true for spread spectrum clock generator sources as well. This will cause higher emission levels that are regulated by the FCC and other regulating bodies, thereby causing additional product costs to comply with these regulations. In situations where two spread spectrum clock generators are operating at the same average frequency and have the same spread spectrum profile, the two sources that are aligned in phase will typically double the output emissions, for a +6 dB increase in those emissions.
It would be an advantage to design a circuit that can supply two different spread spectrum clock generators from a single fixed-frequency source in which the overall emissions are not doubled, but instead are greatly reduced from that typical doubling effect.
Accordingly, it is an advantage of the present invention to provide two spread spectrum clock generator circuits that can operate from a single fixed-frequency source, but do not double their overall emissions when operating in an overlapping frequency situation. it would be another advantage of the present invention to provide two separate SSCG sources that can completely overlap in frequency spectrum, and even have the same spread spectrum profile, while nevertheless reducing their combined emissions to a level that is barely above the emissions of a single spread spectrum clock generator source.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a method for controlling a plurality of spread spectrum clock generator circuits is provided, including the following steps: (1) providing a first spread spectrum clock generator circuit and a second spread spectrum clock generator circuit, and providing a synchronization control logic circuit; (2) controlling the first spread spectrum clock generator circuit so that it outputs a first predetermined plurality of frequencies over a first predetermined time period, as according to a first predetermined spread spectrum profile; (3) controlling the second spread spectrum clock generator circuit so that it outputs a second predetermined plurality of frequencies over a second predetermined time period, as according to a second predetermined spread spectrum profile; and (4) further controlling the second spread spectrum clock generator circuit so that its second predetermined plurality of frequencies at least partially overlaps the first predetermined plurality of frequencies of the first spread spectrum clock generator circuit, while at the same time controlling a frequency difference between the outputs of both the first and second spread spectrum clock generator circuits so that an overall increase in electromagnetic emissions due to a combination of the first and second spread spectrum clock generator circuits is less than 6 dB above the electromagnetic emissions due solely to the first spread spectrum clock generator circuit.
In accordance with another aspect of the present invention, a multiple output spread spectrum clock generator circuit is provided, which comprises a first spread spectrum clock generator circuit and a second spread spectrum clock generator circuit; the first spread spectrum clock generator circuit outputs a first predetermined plurality of frequencies over a first predetermined time period, as according to a first predetermined spread spectrum profile; the second spread spectrum clock generator circuit outputs a second predetermined plurality of frequencies over a second predetermined time period, as according to a second predetermined spread spectrum profile, wherein the second predetermined plurality of frequencies of the second spread spectrum clock generator circuit at least partially overlaps the first predetermined plurality of frequencies of the first spread spectrum clock generator circuit; and a synchronization control logic circuit which controls in real time a frequency difference between the outputs of both the first and second spread spectrum clock generator circuits so that an overall increase in electromagnetic emissions due to a combination of the first and second spread spectrum clock generator circuits is less than 6 dB above the electromagnetic emissions due solely to the first spread spectrum clock generator circuit.
In accordance with a further aspect of the present invention, a multiple output spread spectrum clock generator circuit is provided, which comprises a first spread spectrum clock generator circuit, and a second spread spectrum clock generator circuit; the first spread spectrum clock generator circuit outputs a first predetermined plurality of frequencies over a first predetermined time period, as according to a first predetermined spread spectrum profile; the second spread spectrum clock generator circuit outputs a second predetermined plurality of frequencies over a second predetermined time period, as according to a second predetermined spread spectrum profile, wherein the second predetermined plurality of frequencies of the second spread spectrum clock generator circuit at least partially overlaps the first predetermined plurality of frequencies of the first spread spectrum clock generator circuit; and wherein the first spread spectrum clock generator circuit comprises a frequency synthesizer circuit, the second spread spectrum clock generator circuit comprises a programmable delay chain circuit and a tracking phase locked loop circuit, and the programmable delay chain circuit provides a temporal difference in real time between the outputs of both the first and second spread spectrum clock generator circuits so that an overall increase in electromagnetic emissions due to a combination of the first and second spread spectrum clock generator circuits is less than 6 dB above the electromagnetic emissions due solely to the first spread spectrum clock generator circuit.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.